第12章_多路访问剖析.ppt

* 随着网络利用率的增加，特别是如果有许多站点争用时,可能出现过载情况。此时，网络的吞吐量将急剧下降，信道容量的大部分被CSMA/CD算法耗费掉，只有极小部分用于传送有用的数据。为什么一个共享的以太网要求站点数不得超过1024个，原因就在此。 许多工程师以利用率达到40%作为LAN过载的临界值。利用率较高的LAN将出现高冲突率，传输时间也极可能变化很大（由于退避）。 使用网桥或交换机将LAN分成两个或更多个冲突域将是极其有益的。 As the network utilisation increases, particularly if there are many stations competing to share the bandwidth, an overload condition may occur. In this case, the throughput of Ethernet LANs reduces very considerably, and much of the capacity is wasted by the CSMA/CD algorithm, and very little is available for sending useful data. This is the reason why a shared Ethernet LAN should not connect more than 1024 computers. Many engineers use a threshold of 40% utilisation to determine if a LAN is overloaded. A LAN with a higher utilisation will observe a high collision rate, and likely a very variable transmission time (due to back off). Separating the LAN in to two or more collision domains using bridges or switches would likely provide a significant benefit . * To ensure that all stations start to receive a frame before the transmitting station has finished sending it, Ethernet defines a minimum frame size (i.e. no frame may have less than 46 bytes of payload). The minimum frame size is related to the distance which the network spans, the type of media being used and the number of repeaters which the signal may have to pass through to reach the furthest part of the LAN. Together these define a value known as the Ethernet Slot Time, corresponding to 512 bit times at 10 Mbps or 100 Mbps, and to 4096 bit times at 1000 Mbps. * 注：前10次重发按二进制指数退避算法计算取K值，后6次重发K按第10次（标准以太网为不超过1023） On a busy network, if all stations attempted to retransmit immediately following a collision, then this would certainly result in another collision. Therefore a procedure is required to ensure that there is only a low probability of simultaneous retransmission. The scheme adopted by Ethernet uses a random back-off period, where each node selects a random number, multiplies this by t